Rescue dart for pre-set frac plug and related methods

ABSTRACT

A rescue dart for deployment down a wellbore to dislodge a pre-set frac plug having a plug mandrel from within the wellbore is provided. The rescue dart includes a dart body adapted to be conveyed down the wellbore via fluid flow and a mandrel defeater connected to the dart body and operable to reduce structural integrity of a portion of the plug mandrel until structural failure of the plug mandrel occurs and the frac plug is unset and dislodged. A corresponding system is also provided. The system includes the rescue dart and a pump for pumping fluids down the wellbore. A method of dislodging a pre-set frac plug using the rescue dart is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC § 119(e) of U.S. Provisional Application No. 62/779,180, filed Dec. 13, 2018, entitled “RESCUE DART FOR PRE-SET FRAC PLUG”, the entirety of which is hereby incorporated by reference.

TECHNICAL FIELD

The technical field generally relates to frac plugs, and more particularly to devices and methods for dislodging a frac plug that has pre-set within a wellbore.

BACKGROUND

In multistage fracturing operations, frac plugs can be deployed down a well in order to isolate different segments of the well which are fractured stage by stage in a process commonly known as the “plug and perf” method. However, as the frac plug is run into the well, it can become pre-set at a location upstream of its target location. The pre-set frac plug must be removed before operations can resume. Known methods of dislodging a pre-set frac plug are costly and usually require mobilizing a rig or coil tubing at surface to drill the plug out of the wellbore.

There is thus a need for a technology that overcomes at least some of the drawbacks of what is known in the field.

SUMMARY

According to a first aspect, there is provided a rescue dart for deployment down a wellbore to dislodge a pre-set frac plug comprising a plug mandrel from within the wellbore. The rescue dart includes a dart body adapted to be conveyed down the wellbore to the pre-set frac plug via fluid flow. The rescue dart further includes a mandrel defeater connected to the dart body and operable to reduce structural integrity of a portion of the plug mandrel until structural failure of the plug mandrel occurs and the frac plug is unset and dislodged.

According to a possible embodiment, the rescue dart is shaped and configured to operate in an engaged position where the dart body engages the pre-set frac plug and at least partially extends within an axial passage of the plug mandrel to position the mandrel defeater at a predetermined location along the axial passage.

According to a possible embodiment, the dart body includes a central segment, a dart head and a dart tail, the dart head being provided at a first end of the central segment, and the dart tail being provided opposite the dart head at a second end of the central segment, and wherein when in the engaged position, the dart tail abuts against the pre-set frac plug to prevent further forward movement of the rescue dart and position the dart body with respect to the pre-set frac plug.

According to a possible embodiment, the dart tail is adapted to sit on a ball seat of the pre-set frac plug.

According to a possible embodiment, the dart tail includes rollers positioned and configured to engage the pre-set frac plug to allow rotational movement of the rescue dart.

According to a possible embodiment, the dart tail includes a wiper fin for sweeping debris downhole as the rescue dart travels along the wellbore.

According to a possible embodiment, the wiper fin has an engagement surface for engaging an inner surface of the wellbore to guide the rescue dart along the wellbore.

According to a possible embodiment, the dart tail includes at least one fluid passage for allowing fluid to flow through the dart tail and into the axial passage of the plug mandrel when the rescue dart is in the engaged position.

According to a possible embodiment, the central segment is a central rod sized and configured to allow fluid to flow around it and through the axial passage when in the engaged position.

According to a possible embodiment, the mandrel defeater is positioned and configured to form a restricted passage for restricting fluid flow through the axial passage to at least partially cut the portion of the plug mandrel via fluid abrasion.

According to a possible embodiment, the mandrel defeater extends radially and outwardly from the dart body proximate the dart head for forming the restricted passage.

According to a possible embodiment, the mandrel defeater is tulip-shaped.

According to a possible embodiment, the mandrel defeater is disk-shaped.

According to a possible embodiment, the mandrel defeater is cone-shaped.

According to a possible embodiment, the mandrel defeater is made of a hardened material.

According to a possible embodiment, the material of the mandrel defeater has a hardness which is greater than a hardness of the material of the plug mandrel.

According to a possible embodiment, the central segment is a central tube having an inner conduit in fluid communication with the at least one fluid passage, and the mandrel defeater includes one or more fluid outlet ports extending through a thickness of the dart head for redirecting fluid flow towards the plug mandrel to at least partially cut the portion of the plug mandrel via fluid jetting.

According to a possible embodiment, the fluid outlet ports are angled such that fluids flowing therethrough cause the rescue dart to rotate within the axial passage.

According to a possible embodiment, the dart body and mandrel defeater are a one-piece unit.

According to a possible embodiment, the mandrel defeater includes at least one cutter blade operable between a retracted configuration where the cutter blade can be introduced within the axial passage, and an extended configuration for engaging and at least partially cutting the portion of the plug mandrel.

According to a possible embodiment, the mandrel defeater further includes an actuator adapted to operate the cutter blade in the extended configuration.

According to a possible embodiment, the actuator is adapted to operate the cutter blade when the rescue dart is in the engaged position.

According to a possible embodiment, the actuator is adapted to operate the cutter blade via hydrostatic and/or hydraulic pressure.

According to a possible embodiment, the actuator is a power charge driven actuator or a battery powered actuator.

According to a possible embodiment, the mandrel defeater includes two or more cutter blades positioned radially about the dart body.

According to a possible embodiment, the cutter blades are adapted to cut the plug mandrel radially.

According to a possible embodiment, the cutter blades are adapted to cut the plug mandrel axially.

According to a possible embodiment, the dart body includes an inner passage extending along the central segment and tapering inwardly towards the dart head, and the actuator includes a plunger engageable within the inner passage for pushing the cutter blades outwardly into the plug mandrel.

According to a possible embodiment, the dart tail defines an inner chamber, and the actuator includes a release mechanism adapted to retain the plunger at least partially within the inner chamber when the rescue dart is not in the engaged position.

According to a possible embodiment, the release mechanism includes a shear screw and the actuator further includes a plunger base, the plunger base extending outwardly from the plunger and being retained within the inner chamber by said shear screw, the shear screw is configured to allow forward movement of the plunger base once a pressure threshold is reached for engaging the plunger within the inner passage.

According to a possible embodiment, the inner chamber has an upstream section and a downstream section defined on either side of the plunger base, and a chamber inlet communicating with the upstream section for allowing fluid to flow therein to apply pressure on the plunger base.

According to a possible embodiment, the chamber inlet is substantially sealed to prevent fluid from flooding the upstream section prior to the rescue dart being in the engaged position.

According to a possible embodiment, the chamber inlet is sealed via a rupture disk.

According to a possible embodiment, the chamber inlet is sealed via a secondary plunger extending from the plunger base opposite the plunger.

According to a possible embodiment, the mandrel defeater is adapted to reduce structural integrity of the portion of the plug mandrel via heat.

According to a possible embodiment, the central segment and/or dart head have an interior volumetric capacity, and the mandrel defeater includes heating material located within the capacity, and an actuator adapted to operate the heating material to heat up the plug mandrel.

According to a possible embodiment, the actuator includes an igniter adapted to heat up the heating material, and a firing pin operable to activate the igniter.

According to a possible embodiment, the dart tail defines an inner chamber, and the actuator includes a release mechanism adapted to retain the firing pin within the inner chamber when the rescue dart is not in the engaged position.

According to a possible embodiment, the release mechanism includes a shear screw and the actuator further includes a plunger base on which the firing pin is positioned, the plunger base is retained within the inner chamber by the shear screw, the shear screw is configured to release the plunger base once a pressure threshold is reached for allowing the plunger base to move forward and engage the firing pin with the igniter.

According to a possible embodiment, the inner chamber has an upstream section and a downstream section defined on either side of the plunger base, and a chamber inlet communicating with the upstream section for allowing fluid to flow therein to apply pressure on the plunger base.

According to a possible embodiment, the chamber inlet is substantially sealed to prevent fluid from flooding the upstream section prior to the rescue dart being in the engaged position.

According to a possible embodiment, the chamber inlet is sealed via a rupture disk.

According to a possible embodiment, the chamber inlet is sealed via a secondary plunger extending from the plunger base opposite the firing pin.

According to a possible embodiment, the pre-set frac plug includes an elastomeric element, and the predetermined position of the mandrel defeater is downstream of the elastomeric element.

According to a possible embodiment, the pre-set frac plug includes a slip member and/or a compression member, and the predetermined position of the mandrel defeater is substantially aligned with the slip member and/or compression member.

According to a possible embodiment, the rescue dart further includes a deployment mechanism connectable to a wireline for running the rescue dart down through a wellhead of the wellbore.

According to a second aspect, a system for dislodging a pre-set frac plug comprising a plug mandrel from within a well assembly is provided. The system includes a rescue dart configured to engage and dislodge the pre-set frac plug, and a pump for providing fluid flow down the wellbore to carry the rescue dart to the pre-set frac plug for engagement therewith.

According to a possible embodiment, the pump is located at surface.

According to a possible embodiment, the system further includes a wireline detachably connectable to the rescue dart for running the rescue dart through a wellhead of the well assembly.

According to a possible embodiment, the rescue dart is run down the wellbore exclusively using fluid flow and gravity.

According to a possible embodiment, the rescue dart is as defined above.

According to a third aspect, a method of dislodging a pre-set frac plug comprising a plug mandrel from within a wellbore is provided. The method includes the steps of deploying a rescue dart within the wellbore; pumping fluid within the wellbore for conveying the rescue dart toward the pre-set frac plug via fluid flow; engaging the rescue dart with the pre-set frac plug; and operating the rescue dart to reduce structural integrity of a portion of the plug mandrel until structural failure of the plug mandrel occurs and the frac plug is unset and dislodged.

According to a possible embodiment, the method further includes the step of monitoring pressure variations of the wellbore at surface.

According to a possible embodiment, the method further includes the step of pumping the dislodged frac plug further downhole together with the rescue dart to allow resuming of fracturing operations.

According to a possible embodiment, the rescue dart is as defined above.

According to a possible embodiment, the central segment and/or dart head have an interior volumetric capacity, and wherein the mandrel defeater includes fuel located within the capacity, the mandrel defeater being adapted to release the fuel within the plug mandrel, causing the fuel to react with the fluids within the wellbore to melt at least one component of the plug.

According to a possible embodiment, the mandrel defeater includes a piston engaged within the capacity and movable towards the distal end via fluid flow to inject fuel into the plug mandrel.

According to a possible embodiment, the mandrel defeater includes a metering device adapted to adjust the flow rate of fluid against the piston for controlling an injection rate of fuel within the plug mandrel.

According to a possible embodiment, the metering device is positioned within the dart body uphole of the piston, and forming therewith an internal chamber, the metering device comprising a passage defining a flow path to allow fluid to reach the internal chamber, the passage being shaped and sized to limit a flow rate of fluids therethrough.

According to a possible embodiment, the passage having a tortuous configuration through the metering device to limit the flow rate of fluids.

According to a possible embodiment, the dart head includes a burst disk at a distal end thereof for containing the fuel within the capacity.

According to a possible embodiment, the fuel is a solid rod, liquid and/or powder provided within the capacity.

According to a possible embodiment, the fuel is adapted to react with water.

According to a possible embodiment, the fuel is a mixture of sodium and/or lithium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example frac plug.

FIG. 2 is a partially cut view of the frac plug shown in FIG. 1, showing various components supported by a mandrel.

FIG. 3A is a side view of a rescue dart according to an embodiment, showing the various sections of the rescue dart.

FIG. 3B is a back view of the rescue dart, showing fluid passages provided in a tail portion of the rescue dart.

FIG. 4 is a side view of an embodiment of the rescue dart, showing wiper fins connected to the dart tail.

FIG. 5 is a sectional side view of the rescue dart shown in FIG. 3A engaged with a frac plug, showing a mandrel defeater redirecting fluid flow towards the plug mandrel.

FIG. 6 is a sectional side view of an embodiment of the rescue dart engaged with a frac plug, showing outlet ports aimed at the plug mandrel.

FIG. 7A is a sectional side view of an embodiment of the rescue dart engaged with a frac plug, showing a mandrel defeater provided with cutter blades in a retracted configuration.

FIG. 7B is a sectional side view of an embodiment of the rescue dart engaged with a frac plug, showing a mandrel defeater provided with cutter blades in an extended configuration.

FIG. 8A is a sectional side view of another embodiment of the rescue dart engaged with a frac plug, showing an inner chamber defined in the dart tail for housing an actuator.

FIG. 8B is a sectional side view of the embodiment shown in FIG. 8A, showing the actuator operating the cutter blades for engaging the plug mandrel.

FIG. 9A is a sectional side view of another embodiment of the rescue dart engaged with a frac plug, showing a heating material engaged within an axial passage of the plug mandrel.

FIG. 9B is a sectional side view of the embodiment shown in Figure A, showing an actuator operating the heating material to heat up the plug mandrel.

FIG. 10 is a schematic representation of a system including a rescue dart and a pump, showing the rescue dart within a wellbore travelling towards a frac plug.

FIG. 11 is a sectional side view of another embodiment of the rescue dart engaged with a frac plug, showing a dart head provided with a fuel compartment for housing fuel to be released within the passage of the plug mandrel.

FIG. 12 is a schematic view of a system including a rescue dart mounted to a wireline for operation thereof within the wellbore.

DETAILED DESCRIPTION

As will be described below in relation to various embodiments, a rescue dart for dislodging a plug that has been pre-set within a wellbore is provided. It should be understood that, as used herein, the expression “dislodge”, and any variants thereof with respect to the pre-set frac plug, can refer to the action of removing the plug from a place and/or a position. It should also be understood that the expression “pre-set” refers to a state of a plug that has set in an engaged or operational configuration prior to being in the desired position along the wellbore. Typically, pre-setting occurs when the elastomeric element of the plug has been prematurely expanded and engages with the casing of the wellbore, or when one or more slip members break/malfunction and engage the casing.

Furthermore, the plugs being referred to in the following disclosure are plugs generally used in relation with multistage fracturing operations, commonly referred to as “frac plugs”. However, it is appreciated that the plugs can be any other type of plugs adapted for use in various well operations, such as bridge plugs for example. Traditionally, frac plugs used in multistage fracturing operations include a plug mandrel around which several components are connected for operating and setting the plug. It will be appreciated that the rescue dart may be used to dislodge pre-set frac plugs from various well assemblies, including vertical wells, horizontal wells, slanted wells and/or wells that have various structure features, such as casings and tubulars.

Broadly described, in one implementation, the rescue dart is configured to be conveyed down a wellbore for dislodging a pre-set frac plug from within said wellbore. The rescue dart includes a dart body shaped and configured for being conveyed down the wellbore via fluid flow. It should be understood that gravity can assist the flow of fluid in carrying the rescue dart down the wellbore. Additionally, the rescue dart includes a mandrel defeater connected to the dart body and operable to unset and dislodge the pre-set frac plug, in a manner that will be described hereinbelow.

Referring to FIGS. 1 and 2, an example frac plug 1 is illustrated. As mentioned above, the frac plug 1 includes a plug mandrel 3 supporting multiple plug components. In this example, the frac plug 1 includes an elastomeric element 5 installed between a pair of compression members 7 a-b, which are in turn provided between a pair of slip members 9 a-b. It is appreciated that the frac plug 1 can be operated between an operational (or set) configuration and an unset configuration. It should be understood that, in the set configuration, the compression members 7 a-b apply pressure on either side of the elastomeric element 5, causing it to extend radially and outwardly from the plug mandrel 3 for engaging with an inner surface of the wellbore to position the frac plug 1 along the wellbore. The plug mandrel 3 illustratively has a central bore defining an axial passage 4 extending along a length of the plug mandrel 3 for allowing fluid flow therethrough. The plug mandrel 3 can further include a ball seat 6 defined at an uphole end thereof for receiving a ball thereon in order to seal the axial passage.

With reference to FIGS. 3A to 5, a rescue dart 10 in accordance with possible embodiments is shown. As previously mentioned, the rescue dart 10 is configured to be conveyed down a wellbore and carried therealong until it reaches and engages the pre-set frac plug 1. More specifically, the rescue dart 10 has a substantially elongated dart body 12 shaped and configured to be conveyed down a wellbore via fluid flow. Preferably, the rescue dart is thus conveyed as a free unit that is not connected to a mechanical deployment structure, such as wireline or coiled tubing or the like. In some embodiments, the dart body 12 can be made of a material which can be substantially easy to mill up/out from the wellbore, such as phenolic, composite fiberglass, cast iron or sintered metal, or a substantially disintegrating material such as aluminum and/or magnesium.

The rescue dart 10 includes a mandrel defeater 14 connected to the dart body 12 and being operable to reduce structural integrity of the plug mandrel 3 until structural failure of the plug mandrel 3 occurs, effectively unsetting and dislodging the frac plug 1 from within the wellbore. As will be explained below in relation to various embodiments, the mandrel defeater 14 can be operated via hydrostatic pressure, hydraulic pressure, a power charge or a combination thereof, depending on its nature and mode of operation. However, it will be appreciated that the mandrel defeater 14 can alternatively be operated via any other suitable method.

The rescue dart 10 is configured to operate in an engaged position where the dart body engages the pre-set frac plug 1. For example, when in the engaged position, the dart body 12 at least partially extends within the axial passage 4 of the plug mandrel 3 in order to position the mandrel defeater 14 at a predetermined location along the axial passage 4 (FIG. 5). As such, the mandrel defeater 14 can be operated to reduce the structural integrity of the plug mandrel 3 at a specific or desired location along the axial passage 4. In some embodiments, the predetermined location substantially corresponds with the section of the plug mandrel 3 located downstream/downhole of the elastomeric element 5 of the pre-set frac plug 1. More specifically, the predetermined location can be aligned with the slip member 9 b and/or compression member 7 b of the pre-set frac plug. As such, once structural failure of the plug mandrel 3 occurs proximate the predetermined location, the pre-set frac plug should unset and become dislodged from the wellbore. It should be appreciated that the predetermined location can alternatively be at any suitable location along the axial passage 4, and can be determined based on the particular structure and arrangement of the plug to be dislodged. In some embodiments, the dart body 12 includes a central segment 16, a dart head 18 and a dart tail 20. As seen in FIG. 3A, the dart head 18 is provided proximate a first end of the central segment 16, and the dart tail 20 is provided proximate a second end thereof, opposite the dart head 18. However, it is appreciated that other configurations of the dart body 12 are possible. As best seen in FIG. 5, the dart head 18 is shaped and configured to be inserted within the axial passage 4 of the plug mandrel 3 while the rescue dart 10 is in the engaged position. The dart tail 20 is shaped and configured to engage/abut against the pre-set frac plug 1 in order to prevent further forward movement of the rescue dart 10. As such, the dart body 12 can be arranged in a desired position with respect to the pre-set frac plug 1. It should be understood that the desired position can differ depending on the design of the frac plug 1 and/or the method used to operate the mandrel defeater 14.

Still referring to FIGS. 3A to 5, the dart tail 20 can be adapted to engage an uphole end of the frac plug 1 in order to prevent forward movement of the rescue dart 10 once engaged with the frac plug. For example, the dart tail 20 can be shaped to cooperate with the ball seat 6 of the plug mandrel 3. More specifically, the dart tail 20 can extend outwardly from the central segment 16 (radially and/or at an angle) such that it cannot fully enter within the axial passage 4 of the plug mandrel 3. However, it is appreciated that other configurations are possible, such as having the dart head 18 abut against a portion of the downhole end of the frac plug 1 for example. In some embodiments, the dart tail 20 can be provided with fluid passages 26 for allowing fluids to flow through the dart tail 20 and through the axial passage 4 of the plug mandrel 3 when the rescue dart 10 is in the engaged position. In the illustrated embodiment, the dart tail 20 includes two fluid passages 26, which can have semi-circle shapes, although it is appreciated that any other suitable number and shapes of fluid passages 26 is possible. The fluid passages can be defined as closed passages (i.e., defined exclusively by the structure of the dart tail 20), as in FIG. 3B, or as open passages (i.e., defined by a combination of the structure of the dart tail 20 and the plug mandrel 3) depending on the configuration of the tail 20.

Referring more specifically to FIG. 4, the dart tail 20 can include one or more wiper fins 22 connected thereto and extending therefrom. The wiper fins 22 can be shaped and configured to maintain, or increase, the efficiency of the fluids being pumped down the well. In some embodiments, the wiper fins 22 can be further adapted to sweep and/or drag debris downhole as the rescue dart 10 travels further down the wellbore. It will thus be understood that, once the pre-set frac plug has been dislodged, the wiper fins 22 can help drag the debris created from the collapsed frac plug further downhole, thus allowing a new frac plug to be installed down the wellbore in the desired location. In some embodiments, the dart tail 20 includes a single wiper fin 22 extending outwardly therefrom on all sides (e.g., around 360 degrees) in order to substantially cover the cross-sectional area of the wellbore for dragging debris downhole. Alternatively, the dart tail 20 can include a plurality of wiper fins 22, such as those of FIG. 11, extending from the dart tail 20 in different directions and cooperating with one another to substantially cover the cross-sectional area of the wellbore.

In some embodiments, the wiper fins 22 can respectively include an engagement surface 24 for engaging the inner surface of the wellbore to help guide the rescue dart 10 as it is carried down the wellbore. As such, the dart body 12 can remain substantially aligned with a central axis of the wellbore in order to facilitate at least partial entry of the dart body 12 within the plug mandrel 3. In some embodiments, the wiper fins 22 are made of flexible material such as nitrile rubber, urethane, or foam for example, although it is appreciated that other materials are possible.

In some embodiments, the rescue dart 10 can include a deployment mechanism 25 connectable to a wireline (not shown) configured for running the rescue dart 10 down through a wellhead of the well assembly. More specifically, the deployment mechanism 25 can be releasably connected to the wireline for running the rescue dart 10 below the wellhead and/or down a portion of the wellbore prior to releasing it to allow fluid flow to carry the rescue dart 10 towards the frac plug. However, as noted above, a preferred method of conveying the rescue dart is to carry it as a free body using fluid flow. In various exemplary embodiments of the rescue dart 10, the mandrel defeater 14 can be configured as a fluid-based system for accelerating fluid to impact the plug mandrel 3 at high speeds to enable erosion. As will be described below, the fluid-based mandrel defeater is shaped and configured to restrict fluid flow through the axial passage 4 of the plug mandrel 3, effectively increasing the velocity of the fluid, and redirecting the accelerated flow towards the plug mandrel 3. In some embodiments, the mandrel defeater 14 is made of a hardened material (e.g., steel, brass, aluminium, ceramic, cast iron, tungsten carbide, cobalt alloy, degradable metal composite, etc.) for preventing or reducing wear caused by the fluids being pumped downhole. It should be understood that the material of the mandrel defeater 14 preferably has a hardness which is greater than that of the material of the plug mandrel 3 to prevent structural failure of the rescue dart 10 prior to that of the plug mandrel 3.

Referring back to FIG. 3A to 5, in some implementations, the central segment 16 can be a central rod 28 adapted to extend within the axial passage 4 of the plug mandrel 3 when the rescue dart 10 is in the engaged position. As seen in FIG. 5, the central rod 28 is shaped and configured to allow fluids being pumped downhole to flow around it and through the axial passage 4, i.e., the cross-sectional area of the central rod 28 is less than that of the axial passage 4. The mandrel defeater 14 can be positioned and configured to form a narrow passage 30 for restricting fluid flow through the axial passage 4 such that the fluid flow increases in velocity. The mandrel defeater 14 can be further shaped and configured to redirect the accelerated fluid flow, or jet, towards the plug mandrel 3 in order to at least partially cut an area or portion of the plug mandrel 3 via fluid abrasion, or fluid jetting. It should be understood that the expressions “fluid abrasion” and “fluid jetting” refers to the use of a jet or flow of fluid, such as water, to cut through various materials. Fluid jetting is generally accomplished by using a pressurized high-velocity jet of water, or a combination of water and an abrasive material, although it is appreciated that other fluids, or combination thereof, are possible

In this embodiment, the mandrel defeater 14 is positioned proximate the dart head 18 for redirecting and accelerating the fluids flowing around the central rod 28, although other configurations are possible. In some embodiments, the mandrel defeater 14 can be tulip-shaped for simultaneously redirecting the fluid flow and restricting the axial passage 4. However, it is appreciated that the mandrel defeater 14 can have any suitable shape, or combination of shapes, such as a cone and/or a disk for example. It is further appreciated that the mandrel defeater 14 can extend from the central rod 28 at two or more locations in order to reduce structural integrity of the plug mandrel 3 at several locations.

Now referring to FIG. 6, another embodiment of the rescue dart 10 is shown. In this embodiment, the central segment 16 includes a central tube 32 defining an inner conduit 34 extending therethrough. It is appreciated that the inner conduit 34 of the central tube 32 is in fluid communication with the fluid passage(s) 26 of the dart tail 20 to allow fluid flow therein and towards the dart head 18. In this embodiment, the mandrel defeater 14 includes at least one fluid outlet port 36 extending through a thickness of the dart head 18 so fluid can be expelled towards the plug mandrel 3. The fluid outlet port 36 is shaped and configured to restrict the passage of fluid flow therethrough in order to form a high-velocity flow or jet for cutting the plug mandrel 3.

In this embodiment, the mandrel defeater 14 includes a plurality of fluid outlet ports 36 adapted to form respective jets to impact the plug mandrel 3 at several locations simultaneously. In addition, the fluid outlet ports 36 can be angled in a manner such that the jets exiting the inner conduit 32 via the outlet ports 36 cause the rescue dart 10 to rotate within the axial passage 4. As such, it will be appreciated that the mandrel defeater 14 can be adapted to reduce structural integrity of the plug mandrel 3 by cutting it radially via the spinning jets. In some embodiments, the dart tail 20 can be provided with rollers 37 adapted to abut against a surface of the pre-set frac plug 1, e.g., the ball seat, to facilitate rotational movement of the rescue dart 10 within the axial passage 4.

With reference to FIGS. 7A through 8B, yet another embodiment of the rescue dart 10 is shown. In this embodiment, the mandrel defeater 14 includes at least one cutter blade 38 adapted to be inserted within the axial passage 4, and operable to extend outwardly to contact the plug mandrel 3 to at least partially mechanically cut it. In other words, the cutter blade 38 can be operated between a retracted position (FIGS. 7A and 8A), where the cutter blade 38 can be introduced within the axial passage 4 without contacting the plug mandrel 3, and an extended position (FIGS. 7B and 8B) for engaging the plug mandrel 3.

In some embodiments, the mandrel defeater 14 includes a plurality of cutter blades 38 positioned radially about the dart body 12 such that operating the cutter blades 38 cuts the plug mandrel 3 at a plurality of locations around the periphery thereof. As such, the cutter blades 38 can be configured to at least cut the plug mandrel 3 radially, although it is appreciated that the cutter blades 38 can be adapted to cut the plug mandrel 3 in any suitable manner, such as axially for example. In some embodiments, the cutter blades 38 are fixedly connected to the central segment 16 and/or dart head 18 although it is appreciated that other configurations are possible. Furthermore, the mandrel defeater 14 can be provided with a sleeve (not shown) adapted to cover and retain the cutter blades 38 in the retracted configuration to facilitate insertion of the mandrel defeater 14 (i.e., the cutter blades 38 in this embodiment) within the axial passage 4.

In this embodiment, the mandrel defeater 14 further includes an actuator 40, or actuating assembly, adapted to operate the cutter blades 38 between the retracted and extended positions. The actuator 40 can be configured to only operate the cutter blades 38 when the rescue dart 10 is in the engaged position. As such, untimely extension of the cutter blades 38 can be prevented, which would in turn prevent the rescue dart 10 from effectively engaging the pre-set frac plug 1. As will be described in greater detail below, the actuator 40 can be adapted to operate the cutter blades 38 via hydrostatic pressure, hydraulic pressure, or a combination thereof. Alternatively, the actuator 40 can be a power charge driven (i.e., energetic) actuator, or a battery-powered actuator including a battery and a controller for example.

Still referring to FIGS. 7A to 8B, the central segment 16 can be provided with an inner passage 42, defining a substantially hollow central segment 16. Furthermore, the inner passage 42 illustratively tapers inwardly towards the dart head 18 such that the tapered portion substantially aligns with the location of the cutter blades 38. The actuator 40 can include a plunger 44 shaped and configured to engage the inner passage 42 for pushing the cutter blades 38 outwardly and into the plug mandrel 3. More specifically, the plunger 44 can include a rod-like body shaped and sized to extend within the inner passage 42 and push against the tapering walls as it extends further towards the dart head 18. It is appreciated that the central segment 16 and/or dart head 18 can be provided with slits axially extending therealong for allowing each cutter blade 38 to be displaced outwardly independently from one another.

It should be appreciated that other cutter blades 38 configurations and/or methods of engaging the cutter blades 38 with the plug mandrel 3 are possible. For example, the cutter blades 38 can be independently housed within the central segment 16 and displaceable in a radial direction (i.e., towards the plug mandrel 3). The cutter blades 38 can thus be shaped and configured to be pushed out of the central segment 16 as the plunger 44 extends within the inner passage 44.

In this embodiment, the plunger 44 is positioned uphole of the central segment 16 proximate the dart tail 20. In some embodiments, the actuator 40 can be adapted to retain the plunger 44 in a standby position prior to the rescue dart 10 engaging the pre-set frac plug 1. More specifically, the actuator 40 can include a release mechanism 46 configured to retain the plunger 44 at least partially uphole of the inner passage 42 when the rescue dart 10 is not in the engaged position. The release mechanism 46 is further configured to release the plunger 44 once the rescue dart 10 engages the frac plug 1, as will be explained below. In the embodiments illustrated in FIGS. 7A to 8B, the release mechanism 46 includes a frangible fastener/retainer, such as a shear screw 48, extending through a thickness of the dart tail 20 and within the plunger 44 for retaining the plunger 44 in the standby position proximate the dart tail 20. The shear screw 48 can be configured to release to plunger 44 once a pressure threshold is reached, effectively allowing the plunger 44 to move forward and engage the cutter blades 38 as explained above.

In some embodiments, the dart tail 20 can be shaped and sized to define an inner chamber 50 for housing the actuator 40 while the rescue dart 10 flows down the wellbore to reach the frac plug. In this embodiment, the actuator 40 further includes a plunger base 52 extending radially and outwardly from the plunger 44 towards the walls of the inner chamber 50. As seen in FIGS. 7A to 8B, the shear screw 48 extends through the dart tail 20 and within the inner chamber 50 for retaining the plunger base 52 therein, which in turn retains the plunger 44 from entering the inner passage 42.

As seen in FIGS. 7A and 7B, the dart tail 20 can have a single fluid passage 26 for allowing fluid flow within the inner chamber 50. The fluid passage 26 can be substantially sealed by the plunger base 52 being retained in said position by the shear screw 48. As such, when the rescue dart 10 engages the pre-set frac plug, fluids flowing down the wellbore will accumulate upstream of the rescue dart 10 and push against the surface area of the plunger base 52. In some embodiments, the plunger base 52 can be provided with one or more O-rings 54 to increase the seal of the fluid passage 26 by the plunger base 52, effectively increasing the hydraulic pressure being applied on the plunger base 52 by the flowing fluids. As the pressure increases, the shear screw 48 will eventually break, effectively releasing the plunger base 52 and allowing forward movement thereof for engaging the plunger 44 within the inner passage 42.

Referring more specifically to FIGS. 8A and 8B, in some embodiments, the inner chamber 50 of the dart tail 20 can include an upstream section 50 a and a downstream section 50 b defined on either side of the plunger base 52. In this embodiment, the pressure differential between the upstream and downstream sections 50 a, 50 b pushes the plunger base 52 forward, therefore engaging the plunger 44 within the inner passage 42. In addition, the fluid passage 26 of the dart tail 20 can define a fluid inlet 56 communicating with the upstream section 50 a for allowing fluids to flow therein. Once fluids have filled the upstream section 50 a, hydraulic and hydrostatic pressure will increase and push against the plunger base 52 to assist in engaging the plunger 44 within the inner passage 42. It should thus be understood that, in the present embodiment, the actuator 40 is adapted to operate the cutter blades 38 via a combination of hydrostatic and hydraulic pressure.

In some embodiments, the fluid inlet 56 can be substantially sealed to prevent fluids from flooding the upstream section 50 a prior to the rescue dart 10 having engaged the pre-set frac plug. As seen in FIGS. 8A and 8B, the fluid inlet 56 can be sealed via a secondary plunger 58 extending from the plunger base 52 opposite the plunger 44. The secondary plunger 58 can be shaped and configured to extend through the fluid inlet 56, effectively sealing the inlet and preventing fluids from flowing within the upstream section 50 a. In some embodiments, the secondary plunger 58 can be provided with an O-ring 54 to increase the seal of the fluid inlet 56. It should thus be understood that fluids being pumped down the wellbore will initially push on an exposed surface area of the secondary plunger 58 until it at least partially clears the fluid inlet 56. Fluids will then start to flood the upstream section 50 a for pushing on the plunger base 52, as described above. It is appreciated that the fluid inlet 56 can be sealed using any suitable method and/or apparatus, such as a rupture disk 60 (seen in FIG. 9A) configured to rupture once a pressure threshold has been reached for example.

Now referring to FIGS. 9A and 9B, yet another embodiment of the rescue dart 10 is shown. In this embodiment, the mandrel defeater 14 is adapted to reduce structural integrity of the plug mandrel 3 via the application and/or transfer of heat. The plug mandrel 3 can at least partially melt (e.g., be softened) under the applied heat until structural failure occurs for dislodging the frac plug. In an exemplary embodiment, the central segment 16 and/or dart head 18 can have a common interior volumetric capacity 62 for containing heating material 63. Moreover, the actuator 40 of the mandrel defeater 14 can be adapted to cooperate with the heating material 63 for producing heat to heat up the plug mandrel 3. More specifically, the actuator 40 can include an igniter 64, or igniting mechanism, adapted to heat up the heating material 63, and a firing pin 66 operable to activate the igniter 64.

As seen in FIGS. 9A and 9B, the actuator 40 can include substantially the same release mechanism 46 as previously described for retaining the firing pin 66 within the inner chamber 50 of the dart tail 20 when the rescue dart 10 is not in the engaged configuration. In other words, the firing pin 66 can be connected to the plunger base 52 held in place by one or more shear screws. As such, once the rescue dart 10 engages the pre-set frac plug, fluid flow applies pressure on the actuator 40 (i.e., on the plunger base 52), which breaks the shear screw, and allows the firing pin 66 to move toward the igniter 64 for heating up the heating material 63.

Now referring to FIG. 11, another embodiment of the rescue dart 10 is illustrated. In this embodiment, the common interior volumetric capacity 62 of the central segment 16 and/or dart head 18 can be a fuel compartment 62 provided with fuel releasably packed therein. The fuel compartment 62 can include any of the above-described mechanisms and/or components for containing the fuel until a time it is required to be released into the mandrel of the plug 3, effectively igniting and melting/softening components of the plug 1 to release it. However, in the illustrated embodiment, the fuel is contained within the fuel compartment 62 using a burst disk 61 positioned at a distal end of the dart, and a piston 53 (similar to the above-described plunger base 52 of FIGS. 8a to 9b ) at a proximal end thereof. The piston 53 can be held in an initial position via shear pins/screws 48 until pressure is applied (e.g., via fluid flow) to break the shear pins 48 and allows the piston 53 to move forward, causing the burst disk to effectively burst, thus releasing fuel into the passage of the plug mandrel 3. The fuel can be further sealed within the fuel compartment 62 using sealing elements surrounding the piston 53, such as O-rings for example, or any other suitable sealing element.

The fuel preferably burns on contact with water such that when the burst disk 61 collapses under the pressure and fuel is released, the reaction occurs rapidly such that the energy released from the ignition is maintained in a target area within the plug 1. In some embodiments, the fuel can be a solid rod of fuel material, powder and/or liquid packed within the fuel compartment 62, although other configurations are possible. In the present embodiment, the fuel is a mixture of sodium and/or lithium adapted to react when in contact with the water within the wellbore to melt the components of the plug 1. The fuel can be used to melt the entire plug 1, or targeted components, such as the slip members 9, for example. It is appreciated that melting certain components, instead of the entire plug, can require less fuel to be packed within the fuel compartment 62.

Furthermore, the rescue dart 10 can be provided with a metering device 68 adapted to at least partially control the rate at which fuel is injected into the passage 4 of the plug mandrel 3 (i.e, the rate at which fuel is released from the fuel compartment 62). The metering device 68 can engage the dart 1 within the central segment 16 thereof, uphole of the piston 53, defining therewith an internal chamber 50 similar to the one described in relation with previous embodiments. Furthermore, the metering device 68 can have an orifice, or channel 69, extending therethrough to allow fluid to reach the internal chamber 50. Fluid flows through the passage 69, causing the pressure to build up within the internal chamber 50 until sufficient force is applied on the shear pins 48 to release the piston 53.

The orifice and/or channel 69 of the metering device 68 can be shaped and sized to control the flow rate of fluid therethrough, thus controlling the rate at which the internal chamber 50 is filled and pressurized, which in turn provides some control on the rate at which the piston 53 moves forward along the dart head 18. It should be understood that, based on the rate at which the piston 53 is moved within the dart 10, the injection rate of fuel within the plug passage 4 can also be determined and/or at least approximately predicted. In some embodiments, the channel 69 can have a small diameter and/or have a tortuous configuration through the metering device 68 in order to reduce the flow rate of fluid therethrough. Alternatively, the metering device 68 can be omitted, and fluid flow can be allowed to push directly against the piston 53 for injecting fuel within the plug 1. It should also be appreciated that other configurations, devices, components and/or methods of controlling the flow rate of fluids through various components of the rescue dart 10, such as the metering device 68, the internal chamber 50 and/or the piston 53, are possible and could be used.

It should be appreciated that other embodiments of the rescue dart are possible for dislodging a pre-set frac plug from within a wellbore. For example, in some embodiments, the rescue dart can be shaped and configured for ramming the frac plug in order to break off at least a portion of it to cause structural failure. In this embodiment, the plug mandrel can be weakened using chemicals pumped down with other fluids in order to facilitate breaking the frac plug using the ram rescue dart. The ram rescue dart can have a spherical dart head shaped and sized to extend within the plug mandrel and contact the ball seat with such force so as to break it and/or other components of the plug. Other embodiments can simply include the spherical dart head, or frac ball, dropped within the wellbore and travelling downhole via fluid flow to contact/ram the plug to break at least one component thereof to dislodge it. Various combinations of the previously described embodiments are also possible for effectively dislodging the pre-set frac plug, and each embodiment should not be interpreted as excluding the features and/or characteristics of the other embodiments.

Referring broadly to FIGS. 1 to 11, it should be noted that in the above described embodiments of the rescue dart 10, the dart body 12, or components thereof, and the mandrel defeater 14, or components thereof, can be formed as a one-piece unit (e.g., moulded). For example, the rescue dart 10 embodiment shown in FIG. 5 illustrates the dart body 12 and mandrel defeater 14 being formed as a one-piece unit. Alternatively, the components of the dart body 12 and/or the mandrel defeater 14 can be separately formed and connected to one another via any suitable method and/or fasteners. For example, the rescue dart 10 shown in FIG. 7A illustrates the mandrel defeater 14 including the actuator 40, which is separately formed and positioned within the dart body 12.

It will be appreciated that the various embodiments of the rescue dart described above can be part of a system for dislodging a pre-set frac plug from within the wellbore of a well assembly. As seen in FIG. 10, the system can include a pump 70 configured to provide fluid flow down the wellbore 72 to carry the rescue dart 10 towards the frac plug 1. The fluid flow is further adapted to operate the rescue dart 10 once engaged with said frac plug. As described above, hydraulic and/or hydrostatic pressure from the flowing fluids can operate the mandrel defeater of the various embodiments of the rescue dart. In this embodiment, the pump is located at surface for pumping fluids down the wellbore.

In some embodiments, the system includes a wireline, or wireline assembly, releasably connectable to the rescue dart for running the rescue dart through a wellhead of the well assembly. It is appreciated that the wireline can be useful for running the rescue dart along a first section of the wellbore before releasing it and allowing fluid flow to carry the rescue dart to the frac plug. However, it should also be appreciated that the rescue dart can be run down the wellbore exclusively using fluid flow, or exclusively using the wireline. FIG. 12 shows a representation of a possible embodiment of the rescue dart 10 mounted onto a wireline 80. It should be understood that using the wireline 80 to run the rescue dart in the wellbore 72 can allow retrieval of the rescue dart 10, i.e., the wireline 80 can be pulled out of the wellbore 72 along with the rescue dart 10. In some embodiments, the dart 10 can be retrieved, and possibly along with at least a portion of the frac plug, such that the retrieved components do not need to be drilled out. Furthermore, retrieving the dart 10 can allow at least some components thereof to be reused.

It is known that, when deploying a frac plug downhole, the plug can stop moving prematurely for unknown reasons. The frac plug can therefore become “pre-set”.

In some embodiments, it can be unclear if the frac plug has stopped moving due to debris down the wellbore, or if the frac plug has malfunctioned and set prematurely in the operational configuration. In order to securely position the frac plug down the wellbore, a setting tool can be operated, for example on wireline, to effectively set the plug in its desired position. The wireline can then be pulled from the wellbore, leaving the now fully-set pre-set frac plug in place. Afterwards, the wireline is pulled up through the wellhead of the well assembly and into the lubricator.

At this point, using the above-described rescue dart, and associated system, a method of dislodging a pre-set frac plug from within a wellbore will now be described. First, the rescue dart is deployed within the wellbore. Then, fluids are pumped down the wellbore for conveying the rescue dart toward the pre-set frac plug via fluid flow. Depending on the location within the wellbore where the frac plug is pre-set, the fluid conveyance can be performed to convey the rescue dart short or long distances along the wellbore, within a vertical wellbore section only or down vertical and horizontal sections of the wellbore. Once the rescue dart has reached the pre-set frac plug, it engages the frac plug in a manner such that operating the rescue dart effectively reduces the structural integrity of the plug mandrel. It is appreciated that the structural integrity of the plug mandrel is reduced until structural failure of the frac plug occurs and it becomes unset, and thus dislodged. Afterwards, additional fluids can be pumped down the wellbore for clearing out the debris and pushing the broken frac plug and rescue dart further downhole to allow resuming of fracturing operations.

It is appreciated that placing the rescue dart below the wellhead and deploying it down the wellbore can be done in various ways. For example, following the removal of the wireline from the well, the lubricator can be removed in order to detach the wireline therefrom for attaching the rescue dart to the lubricator instead. The lubricator can then be replaced atop the wellhead for inserting the rescue dart through the wellhead valve and down the wellbore. Once below the wellhead, the pump(s) can be engaged to carry the rescue dart along the wellbore and toward the pre-set frac plug via fluid flow. In some embodiments, the rescue dart can be detachably connected to a weight bar which is, in turn, attached to the lubricator and lowered below the wellhead. Therefore, once fluids are pumped down the wellbore, the rescue dart is detached from the weight bar under the pressure of the fluid flow to be carried toward the frac plug.

The method can further include a monitoring step, where parameters of the wellbore are monitored at surface during operation of the rescue dart. For example, temperature and/or pressure of the wellbore can be monitored. Doing so can help determine the various steps in the dislodging method. For example, an initial variation in the pressure readings can signify that the rescue dart has successfully engaged the pre-set frac plug. Moreover, a subsequent pressure variation can be an indication that the frac plug has been dislodged, indicating that the fracturing operations can be resumed once the debris has been cleared.

In the above description, the same numerical references refer to similar elements. Furthermore, for the sake of simplicity and clarity, namely so as to not unduly burden the figures with several references numbers, not all figures contain references to all the components and features, and references to some components and features may be found in only one figure, and components and features of the present disclosure which are illustrated in other figures can be easily inferred therefrom. The embodiments, geometrical configurations, materials mentioned and/or dimensions shown in the figures are optional, and are given for exemplification purposes only.

In addition, although the optional configurations as illustrated in the accompanying drawings comprises various components and although the optional configurations of the rescue dart as shown may consist of certain geometrical configurations as explained and illustrated herein, not all of these components and geometries are essential and thus should not be taken in their restrictive sense, i.e. should not be taken as to limit the scope of the present disclosure. It is to be understood that other suitable components and cooperations thereinbetween, as well as other suitable geometrical configurations may be used for the implementation and use of the rescue dart, and corresponding parts, as briefly explained and as can be easily inferred herefrom, without departing from the scope of the disclosure. 

1. A rescue dart for deployment down a wellbore to dislodge a pre-set frac plug comprising a plug mandrel from within the wellbore, the rescue dart comprising: a dart body adapted to be conveyed down the wellbore to the pre-set frac plug via fluid flow; and a mandrel defeater connected to the dart body and operable to reduce structural integrity of a portion of the plug mandrel until structural failure of the plug mandrel occurs and the frac plug is unset and dislodged.
 2. The rescue dart according to claim 1, wherein the rescue dart is shaped and configured to operate in an engaged position where the dart body engages the pre-set frac plug and at least partially extends within an axial passage of the plug mandrel to position the mandrel defeater at a predetermined location along the axial passage.
 3. The rescue dart according to claim 2, wherein the dart body comprises a central segment, a dart head and a dart tail, the dart head being provided at a first end of the central segment, and the dart tail being provided opposite the dart head at a second end of the central segment, and wherein when in the engaged position, the dart tail abuts against the pre-set frac plug to prevent further forward movement of the rescue dart and position the dart body with respect to the pre-set frac plug.
 4. The rescue dart according to claim 3, wherein the dart tail is adapted to sit on a ball seat of the pre-set frac plug.
 5. The rescue dart according to claim 3, wherein the dart tail comprises at least one fluid passage for allowing fluid to flow through the dart tail and into the axial passage of the plug mandrel when the rescue dart is in the engaged position.
 6. The rescue dart according to claim 5, wherein the material of the mandrel defeater has a hardness which is greater than a hardness of the material of the plug mandrel.
 7. The rescue dart according to claim 5, wherein the central segment is a central tube comprising an inner conduit in fluid communication with the at least one fluid passage, and wherein the mandrel defeater comprises one or more fluid outlet ports extending through a thickness of the dart head for redirecting fluid flow towards the plug mandrel to at least partially cut the portion of the plug mandrel via fluid jetting.
 8. The rescue dart according to claim 3, wherein the mandrel defeater comprises at least one cutter blade operable between a retracted configuration where the cutter blade can be introduced within the axial passage, and an extended configuration for engaging and at least partially cutting the portion of the plug mandrel.
 9. The rescue dart according to claim 8, wherein the mandrel defeater further comprises an actuator adapted to operate the cutter blade in the extended configuration.
 10. The rescue dart according to claim 3, wherein the mandrel defeater is adapted to reduce structural integrity of the portion of the plug mandrel via heat.
 11. The rescue dart according to claim 10, wherein the central segment and/or dart head have an interior volumetric capacity, and wherein the mandrel defeater comprises heating material located within the capacity, and an actuator adapted to operate the heating material to heat up the plug mandrel.
 12. The rescue dart according to claim 11, wherein the actuator comprises an igniter adapted to heat up the heating material, and a firing pin operable to activate the igniter.
 13. The rescue dart according to claim 10, further comprising an interior volumetric capacity, and wherein the mandrel defeater comprises fuel located within the volumetric capacity, the mandrel defeater being adapted to release the fuel into the plug mandrel, causing the fuel to react with the fluids within the wellbore to melt at least one component of the plug.
 14. The rescue dart according to claim 13, wherein the mandrel defeater comprises a piston engaged within the capacity and movable towards the distal end via fluid flow to inject fuel into the plug mandrel.
 15. The rescue dart according to claim 14, wherein the mandrel defeater comprises a metering device adapted to adjust the flow rate of fluid against the piston for controlling an injection rate of fuel within the plug mandrel.
 16. The rescue dart according to claim 15, wherein the metering device is positioned within the dart body uphole of the piston, and forming therewith an internal chamber, the metering device comprising a passage defining a flow path to allow fluid to reach the internal chamber, the passage being shaped and sized to limit a flow rate of fluids therethrough.
 17. The rescue dart according to claim 16, wherein the passage has a tortuous configuration through the metering device to limit the flow rate of fluids.
 18. The rescue dart according to claim 14, wherein the dart head comprises a burst disk at a distal end thereof for containing the fuel within the volumetric capacity.
 19. A method of dislodging a pre-set frac plug comprising a plug mandrel from within a wellbore, the method comprising the steps of: deploying a rescue dart within the wellbore; pumping fluid within the wellbore for conveying the rescue dart toward the pre-set frac plug via fluid flow; engaging the rescue dart with the pre-set frac plug; and operating the rescue dart to reduce structural integrity of a portion of the plug mandrel until structural failure of the plug mandrel occurs and the frac plug is unset and dislodged.
 20. The method according to claim 19, further comprising the step of monitoring pressure variations of the wellbore at surface.
 21. The method according to claim 19, further comprising the step of pumping the dislodged frac plug further downhole together with the rescue dart to allow resuming of fracturing operations.
 22. The method according to claim 19, wherein the rescue dart is as defined in claim
 1. 